yadahareva virajet tadahareva pravrajet
My translation : Whenever Vairagyam sets in the mind, right away one should go on Sannyasam
i.e the Asrama in which one lived at that point of time, be it Brahmacharya or Garhasthyam, does not matter at all.
We see this happening in the story of sage Shuka. At the time of his Upanayanam ceremony, he gets Vairagyam and immediately sets off for pArivrAjyam. Hence the below celebrated sloka in praise of Suka
yam pravrajantam anupetam apeta-krityam
dvaipāyano viraha-kātara ājuhāva
putreti tan-mayatayā taravo 'bhinedus
tam sarva-bhūta-hridayam munim ānato 'smi
If a person knows that particular Vidya under discussion in the Upanishad, then even if he does not do pitru karma, the paraloka-gati of his parents is not harmed. So it can be taken as an artha Vada in praise of that Vidya. By implication however, an Ajnaani who has not learnt and practised the Vidya, will still need to perform Sraaddha.
While interpreting this passage in the Upanishads, Sankaracharya states that if the son does not perform Sraadha karmam, some obstruction will be created in the way of the pithrus, from enjoying the results of the karma that they did in this world. I don't remember whether Sankaracharya refers to any Sruti texts in support of his statement.
By the way, some of the quotations that Sankaracharya has given, as from the 'Sruti's, couldn't be traced to any extant Vedas or Upanishads. Scholars consider the texts from which these quotes were given, as those that might have been "lost in the course of time".
Sraddha karma is an integral part of Hindu religious beliefs. Like any other 'belief', it might be considered superstitious by non-believers. To each his way 
